Biofilm may be defined as an undesirable accumulation of microorganisms on a surface and in flocculent masses. It is estimated that more than 99% of all the planet's bacteria live in biofilm communities. Biofilm consists of cells immobilized in a substratum, frequently embedded in an organic polymer matrix of microbial origin, which can restrict the diffusion of substances and bind antimicrobials. In flowing aquatic environments, a biofilm consists of a sticky and absorptive polysaccharide matrix encompassing microorganisms. Biofilm bacteria are morphologically and metabolically distinct from free-floating bacteria. Their structural organization is a characteristic feature and distinguishes biofilm cultures from conventional planktonic organisms.
Biofilms create problems for industry from corroding water pipes to computer-chip malfunctions. Any man-made device immersed in an aquatic habitat is susceptible to colonization by microbial biofilm. For example, biofilm may be present on the surfaces of ship bottoms, industrial pipelines, household drains, and artificial hip joints. For the industrial manufacturer, biofilm clusters represent a source of microbial inoculation in a system and may cause plugging problems. In water treatment facilities, the formation of suspended biofilm produces a bulked biological sludge which settles poorly and is difficult to compact in the clarification process. Both non-filamentous and filamentous bulk forms are prevalent in which numerous bacteria permeate the floc. In addition to their role as fouling agents, biofilms may also have adverse effects on people, including altering their resistance to antibiotics and affecting the immune system. Thus, there exists a need in the art for developing effective methods of removing biofilm.
The dynamic nature of biofilms makes it difficult to measure and monitor biofouling. Biofilms often include embedded inorganic particles such as sediments, scale deposits, and corrosion deposits. Moreover, biofilms continuously change in thickness, surface distribution, microbial populations and chemical composition, and respond to changes in environmental factors such as water temperature, water chemistry and surface conditions. Thus, the complexity of biofilms has reduced the effectiveness of treatment and removal strategies.
Even though most microorganisms in industrial systems are associated with biofilm, they have historically received less attention than planktonic microorganisms. However, it has been shown that various biocides are less effective against biofilm than dispersed cells of the same organism. The most common biocides used in biofilm control are pure free halogen donors such as NaOCl and NaOCl/NaOBr. These, however, must be used in high quantities to be effective. In addition, several recent studies evaluating halogen efficacy on biofilms showed an increased disinfection resistance of attached bacteria to free chlorine. Free chlorine treatment at concentrations usually effective against planktonic microorganisms has little effect on the number of attached bacteria or on their metabolic activity. The data indicate that the transport of free chlorine into the biofilm is a major rate-limiting factor, and increasing concentrations did not increase biocidal efficiency. Griebe, T., Chen, C. I., Srinavasan, R., Stewart P., “Analysis of Biofilm Disinfection By Monochloramine and Free Chlorine,” Biofouling and Biocorrosion In Industrial Water Systems (edited by G. Geesey, Z. Lewandowski, and H-C. Flemming), pp. 151-161, Lewis Publishers (1994).
Excessive reactivity of pure free halogen donors was overcome by using bromochlorodimethylhydantoin (BCDMH). The published study by M. Ludyansky and P. Himpler entitled “The Effect of Halogenated Hydantoins on Biofilms,” NACE, Paper 405 (1997), demonstrated higher efficacy on biofilms compared to pure free halogen donors. However, while effective, it is still not an efficient halogen source when applied to biofilm.
Others have attempted to suppress biofilm growth in aquatic systems by using an oxidizing halogen with the addition of adjuvant. U.S. Pat. No. 4,976,874 to Gannon et al., incorporated herein by reference, discloses a method and formulation for the control of biofouling using an oxidizing halogen in combination with a non-oxidizing quaternary ammonium halide. However, this method poses environmental issues.
Thus, the control of biofilm in aquatic systems has typically involved the addition of oxidizing and non-oxidizing biocides to bulk water flow. However, high levels of these expensive chemicals are needed because their effectiveness is rapidly reduced as a result of exposure to the various physical and chemical conditions in specific applications since the concentration of the biocides is considerably reduced by the time the biocides reach the biofilm.